1. Field of the Invention
The present invention relates to methods of calibrating aircraft sensors and, more particularly, to a method for calibrating pressure-sensing altitude sensors of an aircraft while the aircraft is in flight.
2. Description of the Related Art
Recent changes in aircraft flight regulations for flight operations in the North Atlantic Minimum Navigation Performance Specifications (NAT/MNPS) airspace have reduced the minimum allowable vertical distance between two aircraft flying through this airspace from 2,000 feet to 1,000 feet for operation between specified flight levels, currently, Flight Level (FL) 290 and FL 410, inclusive. The Reduced Vertical Separation Minimum, or RVSM, will allow more aircraft to fly more optimal profiles and increase the air traffic capacity through this airspace.
Thus, for safety and air control purposes, it is now more crucial than ever that the aircraft altitude sensors be calibrated with great accuracy, preferably, as often as is possible.
One common type of altitude sensor on an aircraft uses pressure transducers to measure the static atmospheric pressure outside the aircraft. The pressure measurements are converted to the altitude of the aircraft in accordance with government-specified mathematical relations using various electronic or electromechanical means on board the aircraft.
The pressure measurements of these altitude sensors inherently contain various kinds of errors including measurement noise error and bias error. The measurement noise error consists of static source error arising from defects or deformation in the aircraft skin around a static pressure port, quantization error rising from analog to digital conversion, and/or Gaussian measurement noise error. The bias error results from changing weather conditions as the aircraft flies through regions of high or low atmospheric pressure.
Ideally, the altitude sensors are calibrated while the aircraft is in flight since the noise error such as the static source error does not manifest itself until and unless the aircraft is in flight. In the prior art, there are several methods of calibrating aircraft altitude sensors in flight.
One method requires the subject aircraft to tow an aerodynamic cone behind the aircraft at a distance sufficient to avoid the turbulent wake downstream of the aircraft. The cone contains a multitude of reference pressure sensors and recording devices for sensing and recording atmospheric pressures during flight. This method is expensive and highly disruptive in that the aircraft must be removed from service for this test. Furthermore, the altitude measurements of the reference sensors are subject to errors in the range of 50 to 100 feet due to, for example, turbulent air flows induced by the cone and aircraft.
Another known calibration method requires a pacer plane to fly alongside the subject aircraft during a test flight. The pacer plane carries calibrated sensors and recording devices for measuring and recording reference altitude measurements. After the test flight, the sensor measurements of the aircraft are compared with the reference sensor measurements of the pacer plane to determine the magnitude of the errors. This method cannot provide highly accurate static pressure measurements because the reference sensors are also subject to turbulence-induced errors.
In addition to these prior art calibration procedures, there are methods for monitoring the height-keeping performance of the aircraft altimetry system. One height monitoring method requires the aircraft to fly straight and level at a specific flight level over a fixed-based automated height measurement unit, called the Height Monitoring Unit (HMU). The HMU measures the height of the aircraft with a radar. Oftentimes, the aircraft must be diverted from its optimal route for this purpose. This method, however, provides no reference static pressure values for calibrating the aircraft altitude sensors.
Accordingly, there is a need for a convenient, low-cost, high precision method for accurately calibrating aircraft altitude sensors while the aircraft is in flight.